Apparatus for controlling the flow of a cooling medium onto workpieces



June 2, 1970 APPAR Filed Jan. 16, 1968 TAPER VALVE FLOW- OF INITIAL FLOW J. w. cooK 3,514,984

ATUS

FOR CONTROLLING THE FLOW OF A COOLING MEDIUM ONTO WORKPIECES 3 Sheets-Sheet FIG. 3A.

IIIIIIIIIIIT |O203040506O SPEED INCREASE-7o OF INITIAL SPEED TAPER VALVE FLOW- OF INITIAL FLOW FIG. 3B.

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TAPER VALVE FLOW- OF INITIAL FLOW IIIIIIIIIII IQ20304050 TIME-SECONDS ACCEL. TAPER E I I I I I l I I I l I F 20 40 so so I00 :20

TIME SECONDS United States Patent 3,514,984 APPARATUS FOR CONTROLLING THE FLOW OF A COOLING MEDIUM ONTO WORKPIECES John W. Cook, Williamsville, N.Y., assignor to Westmghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 16, 1968, Ser. No. 698,224 Int. Cl. B21b 37/00 US. Cl. 72-7 5 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to apparatus for operating a steel mill, and proposes in particular to increase the cooling medium directed onto a strip of steel, as a function of the acceleration of the strip, and to reduce the flow of the cooling medium, as a function of heat loss resulting from the varying delays of portions of the strip, from the time the strip leaves the heating furnaces.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a control system for reduction rolling of metal or other materials, and more particularly to the apparatus for cooling the rolled metal as a function of the cooling period and the rate of acceleration, to thereby more accurately determine the delivery temperature of the rolled product.

Description of the prior art It is known that the delivery temperature of a hot rolled, metallic strip is a factor in the determination of the metallic quality of the finished strip product. For example, low carbon steel is generally characterized with its best range of metallurgical or other crystalline properties if it is rolled from a hot bar to a delivery temperature in the range of 1500 F. to 1600 F. The particular delivery temperature or range of delivery temperatures of a rolled metal strip is set to optimize a particular property, or to optimize particular groups of properties. By delivery temperature, it is meant to refer to the workpiece or strip temperature as it is delivered from the last hot working point, such as the last roll stand in a hot strip mill.

Since strip delivery temperature is a significant factor in quality control of the finished product, some degree of control over the delivery temperature of the strip is required to maintain the rolled product within an acceptable range of quality. Further, the delivery temperature is dependent upon the measurable or known temperature of the workpiece after the last heating process, and the delay period between the heating process and the last rolling operation. In turn, the period of delay between the last heating operation and the last rolling operation is dependent upon the speed of transit between the furnaces in which the workpieces are heated, and the last rolling stand, and the rolling processes which have been carried out upon the workpiece. Further, the temperature and the metallurgical properties of the workpiece vary along the length of the strip or workpiece. Any nonunitormity of the properties and quality of a rolled strip is largely or substantially brought about because the strip delivery temperature drops from the leading to the trailing end. It may be understood that the workpiece is being continually lengthened as it is rolled, and that the leading edge of the strip will be processed at a point in time before the trailing edge, therefore accounting for a longer period in which the trailing edge may cool and thus be at a lower temperature.

As described in a copending application of the ap- 3,514,984 Patented June 2, 1970 ice plicant, Ser. No. 499,493, filing date Oct. 21, 1965 and entitled Temperature Control System and Method for Operating a Reduction Rolling Mill, now Pat. No. 3,418,834 a method and apparatus has been described for providing an improved control system for regulating the strip delivery temperature in hot strip rolling mills, and in particular for holding the strip delivery temperature substantially constant so as to produce a substantially uniform product quality along the length of the strip. As suggested in this copending application, now Pat. No. 3,418,834 the delivery temperature of the hot strip of steel is maintained at a substantially constant level by accelerating the strip from a first speed toward a higher speed at a moderate rate of acceleration.

In recent years, hot strip mills have been developed and installed that are capable of transporting strips at a speed in the order of 4000 ft./min. In the above-identified copending application, now Pat. No. 3,418,834 the delivery temperature is held constant by maintaining the acceleration rate at modest values in the order of 10 to 20 ft./min./sec. As a result, only extremely long strips can be accelerated at this rate to reach the top speed of the hot rolling mill. For example, if a strip were threaded at 2000 ft./min. and rolled for 600 feet before initiating an acceleration of 15 ft./min./sec., the velocity of the mill would reach its top velocity of approximately 4000 ft./min., 133 seconds after the initiation of the acceleration of the strip. A strip rolled in this manner would have to be approximately 7250 feet long to reach the top speed of the mill.

The limiting factor requiring lower or moderate rates of acceleration is that the temperature of the strip will begin to rise; more specifically, the temperature of the strip will begin to rise when the strip is accelerated greater than a rate of approximately 15 ft,/rnin./sec.,

assuming that customary flow rates of water are used to descale and to cool the strip.

It is therefore an important object of this invention to increase the rates of acceleration of a hot strip millto thereby increase the production rate of the mill.

It is a more particular object of this invention to increase the acceleration rates of a hot strip mill in order that shorter strips may be processed at higher speeds.

Valves to control a cooling medium such as water onto the strip have been used in the prior art. As explained above, the leading edge of the strip is at a higher temperature than the trailing edge and it is necessary to compensate for the temperature decay of the hot strip on the entry side of the rolling mill. Thus the hot strip mills of the prior art have controlled, as by a valve, the flow of a cooling medium such as water, onto the strip by decreasing or throttling down the water flow as the temperature of the strip decreases.

It is a further object of this invention to not only compensate for temperature decreases due to the delay of the trailing portions of the strip entering the mill, but also to control the flow of the cooling medium for changes of temperature of the strip as a function of the acceleration of the strip.

SUMMARY OF THE INVENTION These and other objects are accomplished in accordance with the teachings of the present invention by providing a new and improved apparatus for rolling a material such as steel in which the flow of a cooling medium is so controlled that the fiow may be either increased or decreased. More specifically, the rate of flow of the cooling medium upon the workpiece or strip is decreased in order to compensate for loss of heat from the workpiece as it is being fed into the rolling mill, and the rate of flow of the cooling medium is increased to maintain the delivery temperature of the workpiece approximately con stant as a function of the acceleration rate of the workiece. p DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 shows a schematic diagram of a hot strip, rolling mill arranged and operated in accordance with the teachings of this invention;

FIG. 2 is a schematic diagram of a control circuit for controlling the flow of a cooling medium onto the workpiece processed by the rolling mill of FIG. 1 in accordance with the teachings of this invention; and

FIGS. 3A, 3B and 3C show various curves illustrating the operating principles upon which this invention is based.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, and in particular FIG. 1, there is shown an illustrative embodiment of a hot strip mill for successively reducing the thickness of a slab to form an extended web or strip 84. A plurality of slabs are heated in preparation for being rolled in a plurality of furnaces 12. The slabs are extracted from the furnaces 12 and are disposed upon a plurality of tables 14. The tables 14, in turn, feed the slabs into a roughing mill 16.

The roughing mill 16 includes a plurality of tables 15 disposed between a plurality of stands RS-l to RS6 through which the slabs are transported for gauge reduction. A greater or fewer number of rolling stands can be provided in the roughing mill 16 if desired. Illustratively, the first roughing stands to operate upon the strip may be two high mills including work rolls 19, whereas the last mill stands to operate upon the strip 84 may be a four high stand including back up rolls for pressing the work rolls 19 against the workpiece.

A delay table 22 is disposed between the roughing mill 16 and a finishing mill 26 in order to allow the roughing mill 16 and the finishing mill 26 to be operated independently of each other. It may be understood that as the thickness of the strip 84 is reduced that the leading edge accelerates and is traveling at a higher velocity than the trailing edge. If the leading edge were to be fed into the finishing mill 26 before the trailing edge had left the roughing mill 16, it would be necessary to accurately con trol the velocity of the finishing mill 2'6 with respect to the velocity of the roughing stand RS-6 of the roughing mill 16. To avoid the use of expensive and complicated control circuitry, and in some cases, to control the metallurgical quality, the strip 84 is run out onto the delay table 22 before it is fed into the finishing mill 26. In order to receive the entire strip 84, the delay table 22 must be quite long, and it may require in the order of 20 to 100 seconds to allow the leading and trailing ends of the strip 84 to pass a single point on the table 22.

It may be understood that the time required to deliver the workpiece from the furnaces 12 where the slabs are heated until they pass from the delay table 22, allows the workpieces to be cooled by radiation and conduction. As the workpiece is successively rolled, the strip 84 is lengthened, and the leading portion is separated from the trailing I portion. The leading portion of the strip 84 will be fed into the finishing mill 26 at a point in time significantly before the trailing edge, and as a result will be at a higher temperature than the trailing edge. In particular, the period in which the strip 84 is transported by the delay table 22 allows the entire strip 84, and in particular, the trailing portions thereof to radiate heat and to fall to lower temperatures than the leading portions. A suitable temperature detector 24 such as a pyrometer is disposed at a forward portion of the delay table 22 to measure the temperature of the strip 84 as it is being fed into the finishing mill 26.

The finishing mill 26 includes a plurality of finishing rolling stands FS1 to FS-7 for successively reducing the thickness of the strip 84. A greater or a fewer number of rolling stands may be provided in the finishing mill 26. At each stand location, a pair of work rolls 32 and a pair of backup rolls 34 are provided in a conventional manner. Respective motor drives M1 to M7 are provided at the stand locations to drive the work rolls 32 and to transport the strip 84 through the finishing mill 26. The gauge reduction produced by the various work rolls 32 is set by controlling the size of the respective work roll openings through the application of the well known roll force principle. Illustratively, the drive motors M1-M7 may take the form of suitably rated DC motors. Regulated power controlled by an associated regulator is applied to the motors M1M7 to provide speed and acceleration as described in the above-identified copending application, now Pat. No. 3,418,834. For example, a static thyristor power supply can be employed, and the average voltage applied to the armature of each DC motor can be regulated by thyristor firing angle control to vary the motor speed in response to detected speed error. Since it is desirable that each motor cover its speed range at rated armature voltage, shunt field excitation can be varied in each motor to hold counter EMF substantially constant at the rated value as motor speed varies. For example, respective field regulators (not shown) can be employed to lower or raise motor field strength respectively in response to detected increases or decreases in counter EMF resulting from speed regulator control of armature voltage. Field strength changes resulting in further motor speed change and speed regulator control causes armature voltages to return to rated value. The response of the field regulators is normally made long as compared to that of the speed regulators so that interaction is minimized for system stability. The motor speed regulators are arranged in a conventional manner to control the motors as described. Other motor drives or drive arrangements such as variable frequency AC drives or twin drives can be employed, and other motor speed regulators can be employed in accordance with the principles of the invention.

Further, a crop shear 28 of a type Well known in the art, is provided to trim the leading and trailing edges of the strip 84 so they do not damage the work rolls 32. A plurality of cooling assemblies 40 are disposed between the finishing stands in order to direct a suitable cooling medium such as water onto the strip 84. In particular, the cooling assemblies 40 include a plurality of conduits 42 to which are supplied the cooling medium, as will be explained later. The conduits 42 have a plurality of nozzles 44 for directing the cooling medium onto the strip 84. It may be understood that the cooling assemblies 40 could also serve the function of directing the flow of water onto the strip 84 to remove or to descale the film that has built up on the strip 84. In addition, the cooling of the strip 84 may be effected by directing the cooling medium onto the work rolls 32. This may be accomplished through the use of auxiliary conduits 48 which direct the cooling medium onto the rollers 32. It may be understood that I the rollers 32 absorb heat from the strip 84, which heat is in turn dissipated by directing the cooling medium onto the rollers 32.

A plurality of load cells 38 are associated with each of the finishing stands FS-l to FS-7 to detect the roll force applied to the strip 84. As explained in the aboveidentified application, now Pat. No. 3,418,834 stand gauge control is provided in response to a signal derived from the load cells 38 to control the pressure exerted upon the strip 84. After the strip 84 has been passed through the finishing mill 26 it is wound onto a coiler 54 in a manner well known in the art. As shown in FIG. 1, suitable temperature detectors 50 and 52 shuch as pyrometers are disposed to detect the temperature of the strip 84 at selected points within the finishing mill 26.

Referring now to FIG. 2, there is shown a control system 56 for regulating the flow of the cooling medium from the cooling systems 40 and the auxiliary conduits 48 as a function of the delay time of a particular portion of the strip 84, and also as a function of the rate of acceleration of the strip 84 through the finishing mill 26. The overall mill control preferably is provided by a process computer system 58 suitably designed and programmed to provide the degree of process control desired. That portion of the operation of the computer system 58 related to the control of the flow of cooling medium will be explained in particular. A data input device 60 such as a commercially available card reader provides data relating to the dimensions and weight of each slab being processed by the mill into a storage facility within the computer system 58. Input process variables to the computer system 58 may include the entry temperature of the strip 84 into the finishing mill 26 as derived from the temperature detector 24, and the temperature of various portions of the strip 84 as it is being processed upon by the finishing stands as supplied by the temperature detectors 50 and 52. Further, the position of the strip 84 may be detected by the load cells 38 and this information is fed into the computer system 58. As explained above, the leading portions of the strip 84 are at a higher temperature than the trailing portions due to the difference in delays encountered from the furnaces 12 to the finishing mill 26. Upon the basis of the dimensions of the individual slabs and the entry temperature of the strip, the computer system 58- will calculate the rate of temperature decrease for remaining portions of the strip 84. An output signal will be derived from the computer system 58 and applied to a time taper control circuit 64. The signal applied to the circuit 64 will be indicative of when the leading edge of the strip 84 is fed into the finishing mill 26 as Well as the rate of decrease of the temperature of the strip 84. It is noted that this information could be fed directly into the time taper control circuit 64 by an operator or that the temperature detector 24 could be applied directly to the time taper control circuit 64. The time taper control circuit 64 functions to set the rate at which strip cooling water is decreased at a function of time to allow holding essentially a constant strip finishing temperature at a constant mill speed.

In a manner described in the above-identified copending application, now Pat. No. 3,418,834 the computer system 58 controls the drive motors M1 to M7, and more particularly the acceleration of the drive motors M1 to M7 and therefore the strip 84 through the finishing mill 26. In addition, the computer system 58 applies a signal to an acceleration taper control circuit 62 indicative of the rate of acceleration provided to the strip 84, and of the beginning and duration of the acceleration of the strip 84. The acceleration taper control circuit 62 in turn provides a signal indicative, as by amplitude, of the rates of acceleration to a summing amplifier 66. More specifically, the accelleration taper control circuit 62 operates to increase the rate of the cooling medium as a function of the rate of acceleration of the strip 84 through the mill 26. As shown in FIG. 2, the time taper control circuit 64 likewise provides an input signal to the summing amplifier 66 indicative of the necessary compensation due to the temperature differences of various portions of the strip 84 as it enters the finishing mill 26. It is noted that provision may be made for allowing the operator to set the acceleration rate into the circuit 62. The summing amplifier 66 sums the signals derived from the circuits 62 and 64, and applies a resultant output signal to a power supply 68. It is noted that there will be an acceleration rate every workpiece where the time taper rate will be equal and opposite to the acceleration taper rate. Under this condition, there exists no net change in the flow of the cooling medium. However, where the actual acceleration rate is less than the equilibrium rate, the water will be decreased, whereas when the acceleration rate is higher than the equilibrium rate, the rate of flow of the cooling medium will be increased. In turn, the power supply 68 applies a driving signal to a valve motor 70. As shown in FIG. 2, the motor 70 is connected to a taper valve 80, which controls the rate of flow of a cooling medium such as water onto the strip 84. Illustratively, the motor 70 may be a DC motor whose velocity, and therefore position, is dependent upon the voltage applied thereto by the power supply 68. Thus, the degree of closure, or position of the taper valve 80, is dependent upon the sum of the signals derived from the acceleration taper control circuit 62 and the time taper control circuit 64. A suitable cooling medium, such as water, is derived from a water supply 78, the flow of which is controlled by the taper valve 80. Illustratively, the cooling medium may be conveyed from the taper valve by conduits to an on-olf valve 82, and then to the plurality of conduits 42 and 48. Each of the conduits 42 and 48 have a plurality of nozzles for directing the cooling medium in the desired distributions onto the strip 84 and the rollers 32. The on-off valve 82 is operated to turn on the flow of the cooling medium, when the leading edge of the strip 84 reches the first of the cooling assemblies 40 and to turn off the flow of the cooling medium, when the trailing edge of the strip 84 passes the last assembly 40. In large strip mills, there could be more than one on-off valve and also more than one taper valve. In the illustrative embodiment of FIG. 2, the taper valve 80 is not used to turn the system on or off. The valve 80 is turned to its reset position to determine the initial flow of the cooling medium. It is noted that a single valve could perform the functions of the valves 80 and 82.

The position of the valve motor 70, and therefore, the degree of closure of the taper valve 80, is detected by a suitable position detector 72 such as a shaft encoder. The detector 72 in turn provides a feedback signal to a reset position control circuit '74. The reset position control circuit 74 provides a feedback error signal to the summing amplifier 66 which compares the error signal with the summed signal applied to the power supply 68 to thereby accurately control the position of the motor 70 and the taper valve 80. Further, the reset position control circuit 74 is responsive to a signal derived from the computer system 58, which signal indicates the passing of the trailing edge of the strip 84. For example, when the trailing edge of the strip 84 passes the finishing stand FS7, a signal is applied to the reset position control circuit 74, which in turn applies the appropriate signal to the motor 70 to return the taper valve 80 to its initial position. It is noted that in the alternative the operator could reset the position control circuit 74.

Referring now to FIG. 3B, there is shown curves representing the rates of decrease required to compensate for strip heat loss as a result of varying delays of the trailing portions of the strip 84 to allow the strip to be rolled at constant speed and constant temperature. As shown in FIG. 3B, the rate of flow of the cooling medium through the taper valve 80 is decreased due to the fact that the trailing portions of the strip 84 will be at a lower temperature than the leading portions of the strip 84. It is noted that the rate of decrease may be dependent upon the size and the type of metal or material being rolled as indicated by the various curves A, B and C. In accordance with the teachings of this invention, the time taper control circuit 64 provides a signal to the summing amplifier 66 which is proportional to the rate of decrease as shown in FIG. 3B.

Referring now to FIG. 3A, there is shown the rates of increase of the flow of the cooling medium as controlled by the valve 80 as a function of the percent of acceleration from the initial speed of the strip 84. The various curves shown in FIG. 3A are a function of the size of the strip 84 and also the material of which the strip is made. The particular rate of flow of the cooling medium through the valve 80 is controlled by the acceleration taper control circuit 62. More specifically, the circuit 62 provides a signal which is proportional to the desired rate of increase of the taper valve flow to the summing circuit 66. It is a significant aspect of this invention that the rates of increase and decrease respectively derived from the acceleration taper control circuit 62 and the time taper control circuit 64 are summed by the summing amplifier 66 to provide a signal which controls the movement of the drive motor 70, and therefore, the rate of opening and closing (increase or decrease flow of the cooling medium) of the taper valve 80. The summing amplifier 66 is responsive to the circuits 62 and 64 to increase and to decrease the rate of flow of the cooling medium directed onto the strip 84 and onto the work rollers 32. More specifically, the rates of increase and decrease are summed to provide a resultant rate at which the water flow is adjusted i.e., either decreased to compensate for the loss of temperature of the trailing portions of the strip 84 or increased to compensate for temperature increase caused by the acceleration of the strip through the mill 26.

The curves shown in FIGS. 3A and 3B are illustrative.

There could be an infinite number of curves within prescribed boundary conditions. The selected curve on FIG. 3B will be determined primarily by the thickness of the strip 84 on the table 22 and the temperature of the strip on the table 22. The chosen curve of FIG. 3A is determined by the same two factors and in addition, the

mill acceleration rate.

Referring now to FIG. 3C there is shown illustratively how the taper valve 80 will be controlled at the rate of curve A of FIG. 3B and at the rate of curve E in FIG. 3A. Illustratively, the strip 84 may be run at a velocity of 2000 ft./ min. for 25 seconds; then, the strip 84 is accelerated at a rate of 25 ft./min./sec. to 3000 ft./min. and then during the third step, run at a velocity of 3000 ft./ min. for 45 seconds. The taper valve 80 would initially be set to decrease for 25 seconds at the rate of curve A, at FIG. 3B. When the acceleration of the strip 84 is initiated an increasing rate as determined from the curve B of FIG. 3A would be summed with the A curve of FIG. 3B, and the valve 80 would be operated at the resultant or summed rate of increase as shown in FIG. 3C. it is particularly noted that the rate of water flow is increased and continues to increase throughout the acceleration of the strip 84. When the acceleration of the strip is stopped, the rate as derived from curve E of FIG. 3A will be removed from the summing amplifier 66, and at that time, the rate of flow as controlled by the valve 70 will be tapered at the original rate of curve A of FIG. 3B.

Thus, there has been shown and described apparatus for controlling the rate of flow of a cooling medium. More specifically, a valve or other means is operated to increase the flow of the cooling medium as a function of the acceleration of the workpiece (or strip) and to decrease the flow as a function of the temperature loss due to the varying delays of the leading and trailing portions of the strip.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrated and not in a limiting sense.

I claim as my invention:

1. A control system for a rolling mill including a plurality of stands, motor means associated with said stands for directing said strip through said stands, and control means for controlling the rate of flow of a cooling medium for removing heat from said strip, said control means controlling the flow of said cooling medium as a function of the rate of acceleration of said strip by said motor means and of the time of relay of various portions of said strip entering said mill.

2. A control system as claimed in claim 1, wherein said control means increases said rate of flow as a function of the acceleration of said strip and decreases said rate of flow as a function of the delay time of various portions of said strip entering said mill.

3. A control system as claimed in claim 1, wherein said control means includes a valve for increasing and decreasing the flow of said cooling medium and a motor for driving said valve.

4. A control system as claimed in claim 3, wherein said control means further includes first signal means for providing a first signal proportional to the rate of acceleration of said strip, second signal means for providing a second signal proportional to the time of delay of various portions of said strip entering said mill, and third signal means for summing said first and second signals to provide a third signal for controlling said motor.

5. A control system as claimed in claim 4, further including flow means for detecting the rate of flow through said valve, and reset means disposed between said third signal means and said flow means for resetting said valve to its initial condition to receive another of said strips.

References Cited UNITED STATES PATENTS 1,951,426 3/1934 Littler 7213 2,095,430 10/1937 Biggert 72-201 2,319,309 5/1943 Emigh 7213 3,212,309 10/1965 Wilson 72-13 X 3,364,713 1/1968 Fukuda et a1. 72201 MILTON S. MEHR, Primary Examiner US. Cl. X.R. 72-13, 201, 202 

